Immortal jellyfish (turritopsis nutricula)
That isn’t hyperbole – it really is immortal. After reaching sexual maturity, this jellyfish is able to reverse its aging process and become a polyp again. The ability to reverse the life cycle is probably unique in the animal kingdom, and allows the jellyfish to bypass death, rendering the Turritopsis nutricula biologically immortal. Lab tests showed that 100% of specimens reverted to the polyp stage.